1. Field of the Invention
The present invention relates generally to an antenna mirror surface measuring/adjusting apparatus for measuring an accuracy of a mirror surface of a reflection mirror antenna used in a high-frequency band, or measuring and adjusting the mirror surface thereof, and more particularly to an antenna mirror surface measuring/adjusting apparatus of a large-aperture radio telescope used for an observation in millimeter waves and sub-millimeter waves.
2. Description of the Related Art
An antenna mirror surface measuring/adjusting apparatus in the prior art will be explained referring to the drawings. For example, FIGS. 31, 31A illustrate a construction of the conventional antenna mirror surface measuring/adjusting apparatus disclosed in "Surface-Error Measurement of a 45 m Radio Telescope Using Radio Holographic Metrology," written by Masato Ishiguro, Kohichiro Morita, Saeko Hayashi, Gohtoku Masuda, Takashi Hirushii, Shinichi Betsudan, pp.69-74, No.5, Vol. 62, 1988, Technical Report of Mitsubishi Electric Co.
Referring to FIGS. 31, 31A, there are illustrated a principal reflection mirror 1 of a test antenna which is an object for a mirror surface measurement, mirror surface panels 1a into which the mirror surface is segmented, an actuator 1b for changing an offset or an inclination of the mirror surface panel 1a, and a back structure 1c for supporting the mirror surface panel 1a and the actuator 1b.
Referring again to FIGS. 31, 31A, the numeral 2 designates a geostationary satellite, 3 represents a transmission antenna of which a bore site direction is aligned with a direction of the test antenna mounted on the geostationary satellite 2, and 4 denotes transmitting radio waves radiated from the transmission antenna 3. There are also shown a receipt-oriented primary focal horn 5 for receiving the radio waves after reflected by the principal reflection mirror 1 of the test antenna and converged, a receiver 6 supplied with the electricity from the receipt-oriented primary focal horn 5, a two-dimensional radiation pattern receiving signal 7 gained from the receiver 6, and antenna position angle signal 8 for biaxially changing a position of the antenna in order to obtain the radiation pattern receiving signal 7, a radio holography arithmetic processor 9 for calculating an aperture surface distribution by Fourier transform from the radiation pattern receiving signal 7 and from the antenna position angle signal 8, an actuator controller 10 for controlling the actuator 1b for driving the mirror surface panel 1a, and a reference antenna 11 serving as a base of phase.
Note that the number of the mirror surface panels 1a constituting the principal reflection mirror 1 is 600 in the case of a 45 m radio telescope installed in the Nobeyama National Astronomical Observatory, and 36 in the case of a 10 m antenna for millimeter wave interferometer in the Nobeyama National Astronomical Observatory.
In the antenna mirror surface measuring/adjusting apparatus shown in FIG. 31, the radio waves are used for measuring ruggedness on the principal reflection mirror 1 of the test antenna. A position of a transmission source is set well far from the test antenna as in the case of the geostationary satellite 2. A transmission source might be provided on the ground at a far distance instead of the geostationary satellite 2. In such a case, however, there might be selected such a topographic feature as to reduce an influence of the reflection on the ground surface. A radiation pattern of the test antenna is obtained by receiving the transmitting radio waves 4 while two-dimensionally changing the position of the test antenna.
With this operation, the measurement is done, wherein the two-dimensional radiation pattern receiving signal 7 is paired with the antenna position angle signal 8 representing the position of the test antenna. The radio holography arithmetic processor 10 executes an arithmetic process such as fast Fourier transform by utilizing the fact that a relationship between the two-dimensional radiation pattern and the aperture surface distribution is expressed with Fourier transform, and the aperture surface distribution of the test antenna is thus calculated.
By the way, it is required that an accuracy of the mirror surface be on the order of under 1/20 a wavelength in use if considered in terms of an antenna gain. Even in the case of a large aperture, a higher accuracy of the mirror surface must be attained as the wavelength in use becomes shorter ones such as millimeter waves and sub-millimeter waves. Therefore, a measurement frequency must be increased in order to measure the mirror surface at the still higher accuracy. In the transmission radio waves 4 of the geostationary satellite 2 shown in FIG. 31, however, the frequency is limited. Hence, there arises such a problem that the measurement accuracy can not be enhanced because of the measurement frequency being low.
Further, if the transmission source is provided on the ground, as already described in the discussion on the prior art example, the measurement accuracy is restricted due to the influence of the reflection on the ground surface. Moreover, in the case of an outdoor measurement, the measurement accuracy is restricted depending on a measurement environment such as changes in wind, sunlight and temperature. For example, the radiation pattern might be varied by a fluctuation in phase due to the atmospheric air and the shake of the principal reflection mirror due to the wind, resulting in an error of the measurement. Furthermore, in the measurement of the mirror surface using the radio wave holography, if the number of sample points for the measurement is increased for enhancing a resolution of the measurement, a measurement time increases, and the temperature might change during the measurement. Therefore, a configuration of the principal reflection mirror of the test antenna might differ depending on the positions of the sample points, resulting in the error of the measurement. A problem is that the measurement accuracy can not be enhanced in the outdoor measurement of the mirror surface using the radio wave holography.
When measuring the mirror surface indoors, a two-dimensional radiation field must be measured by performing a mechanical scan of a probe on a flat surface, a cylindrical surface and a spherical surface. A scan range is taken wider than the test antenna, and hence, in the case of the large-aperture antenna, it is difficult to precisely scan such a broad range, and the measurement accuracy is restricted by a scan accuracy of the probe. Then, a problem is that the measurement accuracy can not be enhanced.